James Bottomley wrote:
On Sun, 2008-01-20 at 12:56 -0800, Greg KH wrote:
On Sun, Jan 20, 2008 at 11:27:29AM +0100, Hans de Goede wrote:
Hi all,
This patch sets the last_sector_bug flag to 1 for all USB disks. This is
needed to makes the cardreader on various HP multifunction printers work.
Since the performance impact is negible we set this flag for all USB disks
to avoid an unusual_devs.h nightmare.
Oh great, now my "working just fine" USB devices, which happen to have
data in the last sector, suddenly stop working.
That's not acceptable :(
I don't see how this will happen, might you not be confusing this change
(which allows access to the last sector, just insists that it be
accessed by a single sector read) with US_FL_FIX_CAPACITY which is for
devices that report having one more sectors than they actually have and
therefore adjusts the access limits down by one?
Let me try to explain some more, the scsi-sd patch, which goes hand in hand
with this adds a last_sector_bug flag, which is indeed a different flag form
the fix_capacity flag. Both deal with with what are (in case of the
last_sector_bug flag probably) off by one bugs.
The fix_capacity flag is for devices which report their last sector is N (with
sectors numbered from 0 - N) but in reality / they mean they have N sectors,
so their last sector really is N - 1.
The last_sector_bug flag is for a bug (sofar only seen in HP multifunction
printers with cardreader when using an sdcard) where the reader will cease to
function (returns error codes in response to each and every command) after one
has attempted to read the last sector in a read larger then 1 sector. To be
clear an example lets say an example disk has 256 sectors, numbered 0 - 255.
Then the following reads will all cause the reader to go into borked mode:
16 sectors starting at 240
8 sectors starting at 248
2 sectors starting at 254
Yet the last sector can still be read without problems the following read:
1 sector starting at 255
So what the scsi-sd part of these 2 patches does it adds a last_sector_bug
flag, which when set will cause the layer to split a read like this:
16 sectors starting at 240
Into:
15 sectors starting at 240
1 sector starting at 255
Since reading the last sector is a rare occurence (but one which does happen
every time when determining the partition table, triggering the bug on every
card insert). and since there are a lot of different HP printer models ( and
cheap usb card readers are notoriously buggy so other cardreaders might be
affected too), Matthew Dharm (the usb-storage maintainer) thought it best to
enable this for all devices.
Regards,
Hans
-
To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
